This invention relates to power amplifier circuits, and more specifically to a linear amplifier which minimizes power loss in its output devices.
It is well known that the power supply for an amplifier which directly drives a load must have a supply voltage at least slightly greater than the maximum output voltage required by the load. The difference between the supply voltage and the instantaneous load voltage appears across the amplifier's output devices. Because load current also flows through the output devices, there is consequent power loss through them. It is well understood that the efficiency of the amplifier increases as the load, or output, voltage approaches the supply voltage. In most amplifier applications, however, the load voltage varies and is less than maximum the majority of the time. Therefore, efficiency suffers the majority of the time as well. The common technique of using a class A amplifier powered by a supply voltage high enough to accommodate the highest expected output voltage exhibits this characteristic.
It has been recognized in the prior art that for an amplifier to drive a load efficiently at both high and low output voltages, the amplifier must be able to supply load current from a low voltage supply for low output voltages and from a higher voltage supply only during higher output voltages. This reduces the voltage drop across the amplifier output devices and, thus, improves its efficiency.
Prior art multi power supply amplifier circuits are disclosed in an article by Jerome Leiner in "Electronics Magazine", Nov. 9, 1978, page 114; U.S. Pat. No. 3,772,606; U.S. Pat. No. 3,961,280; and U.S. Pat. No. 4,319,199. However, even these approaches have problems with supply crossover distortion and turn-on and turn-off delays in the transistors at high speed, that is, at speeds in excess of, for example, 25 kHz.